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<div><strong class="selflink"><span class="t-lines"><span>alignas</span></span></strong></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div>
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<p>Appears in the <a href="declarations.html" title="c/language/declarations">declaration</a> syntax as one of the type specifiers to modify the <a href="object.html#Alignment" title="c/language/object">alignment requirement</a> of the object being declared.
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="alignas.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="alignas.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="alignas.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="alignas.html#Keywords"><span class="tocnumber">4</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="alignas.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="alignas.html#References"><span class="tocnumber">6</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="alignas.html#See_also"><span class="tocnumber">7</span> <span class="toctext">See also</span></a></li>
</ul>
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<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span>
</h3>
<table class="t-sdsc-begin">

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<td> <code><b>_Alignas</b></code> <code><b>(</b></code> <span class="t-spar">expression</span> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span>
</td>
</tr>



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<td colspan="10" class="t-sdsc-sep">
</td>
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<tr class="t-sdsc">
<td> <code><b>_Alignas</b></code> <code><b>(</b></code> <span class="t-spar">type</span> <code><b>)</b></code>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span>
</td>
</tr>



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<td colspan="10" class="t-sdsc-sep">
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<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">expression</span>
</td>
<td> -
</td>
<td>  any <a href="constant_expression.html" title="c/language/constant expression">integer constant expression</a> whose value is a valid <a href="object.html#Alignment" title="c/language/object">alignment</a> or zero
</td>
</tr>
<tr class="t-par">
<td>  <span class="t-spar">type</span>
</td>
<td> -
</td>
<td>  any <a href="compatible_type.html#Type_names" title="c/language/type">type name</a>
</td>
</tr>
</table>
<p>This keyword is also available as convenience macro <a href="../types.html" title="c/types"><tt>alignas</tt></a>, available in the header <code>&lt;stdalign.h&gt;</code>.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span>
</h3>
<p>The alignas specifier can only be used when declaring objects that aren't <a href="bit_field.html" title="c/language/bit field">bit fields</a>, and don't have the <a href="storage_duration.html" title="c/language/storage duration">register</a> storage class. It cannot be used in function parameter declarations, and cannot be used in a typedef.
</p>
<p>When used in a declaration, the declared object will have its <a href="object.html#Alignment" title="c/language/object">alignment requirement</a> set to 
</p>
<div class="t-li1">
<span class="t-li">1)</span> the result of the <span class="t-spar">expression</span>, unless it is zero</div>
<div class="t-li1">
<span class="t-li">2)</span> to the alignment requirement of <span class="t-spar">type</span>, that is, to <span class="t-c"><span class="mw-geshi c source-c">alignof<span class="br0">(</span>type<span class="br0">)</span></span></span>
</div>
<p>except when this would weaken the alignment the type would have had naturally.
</p>
<p>If <span class="t-spar">expression</span> evaluates to zero, this specifier has no effect.
</p>
<p>When multiple alignas specifiers appear in the same declaration, the strictest one is used.
</p>
<p>Alignas specifier only needs to appear on the <a href="declarations.html#Definitions" title="c/language/declarations">definition</a> of an object, but if any declaration uses alignas, it must specify the same alignment as the alignas on the definition. The behavior is undefined if different translation units specify different alignments for the same object.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span>
</h3>
<p>As initially published, C11 does not allow alignas specifiers in structure and union members; this was corrected by DR 444.
</p>
<p>In C++, the <code>alignas</code> specifier may also be applied to the declarations of class/struct/union types and enumerations. This is not supported in C, but the alignment of a struct type can be controlled by using alignas in a member declaration (as of DR 444).
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span>
</h3>
<p><a href="../keyword/_Alignas.html" title="c/keyword/ Alignas"><tt>_Alignas</tt></a>
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span>
</h3>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdalign.h&gt;</span>
<span class="co2">#include &lt;stdio.h&gt;</span>
<span class="co1">// every object of type struct sse_t will be aligned to 16-byte boundary</span>
<span class="co1">// (note: needs support for DR 444)</span>
<span class="kw1">struct</span> sse_t
<span class="br0">{</span>
  alignas<span class="br0">(</span><span class="nu0">16</span><span class="br0">)</span> <span class="kw4">float</span> sse_data<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// every object of type struct data will be aligned to 128-byte boundary</span>
<span class="kw1">struct</span> data <span class="br0">{</span>
  <span class="kw4">char</span> x<span class="sy4">;</span>
  alignas<span class="br0">(</span><span class="nu0">128</span><span class="br0">)</span> <span class="kw4">char</span> cacheline<span class="br0">[</span><span class="nu0">128</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// over-aligned array of char, </span>
                                    <span class="co1">// not array of over-aligned chars</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"sizeof(data) = %zu (1 byte + 127 bytes padding + 128-byte array)<span class="es1">\n</span>"</span>,
           <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> data<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
 
    <a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignment of sse_t is %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw1">struct</span> sse_t<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
 
    alignas<span class="br0">(</span><span class="nu0">2048</span><span class="br0">)</span> <span class="kw1">struct</span> data d<span class="sy4">;</span> <span class="co1">// this instance of data is aligned even stricter</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">sizeof(data) = 256 (1 byte + 127 bytes padding + 128-byte array)
alignment of sse_t is 16</pre></div></div> 
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span>
</h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd>
<ul><li> 6.7.5 Alignment specifier 
</li></ul>
</dd></dl>
<dl><dd>
<ul><li> 6.2.8 Alignment of objects (p: 48-49)
</li></ul>
</dd></dl>
<dl><dd>
<ul><li> 7.15 Alignment &lt;stdalign.h&gt; (p: 268)
</li></ul>
</dd></dl>
</div>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see">
<span><a href="../../cpp/language/alignas.html" title="cpp/language/alignas">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>alignas specifier</span></span>
</div>
</td>
</tr>

</table>

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